Manufactures of semiconductors normally use diffusion furnaces to dope silicon crystals or silicon polycrystalline substrates with Group III or Group V elements, such as boron and phosphorus, to manufacture semiconductors. The dopants, as well as passivating compounds, are supplied by gas streams flowing into the diffusion furnace. Typical gases include hydrogen, hydrogen chloride, nitrogen, oxygen and silane. It may be appreciated that the concentration of doping material in the silicon must be controlled very precisely to provide semiconductors having predictable and uniform electrical characteristics. Thus, it is necessary to meter precisely the amount of gas which is supplied to the diffusion furnace. Furthermore, two or more gases are sometimes reacted in the furnace. In this event, precise stoichiometric proportions must be maintained for gas flow measurement.
The prior art teaches a number of electronic systems having bridges fed from constant current generators. The bridges usually have a pair of resistors positioned within or in good heat conduction relation to the main gas stream or a branch thereof, one resistor being disposed upstream from the other. When current is supplied to the bridge, both the upstream and downstream resistors generate heat, which is transferred to the gas flowing past the resistors. The upstream resistor is cooled more than the downstream resistor, causing a shift in the voltage at the junction between the upstream and downstream resistors. This junction voltage shift is amplified and compared with a set point voltage to produce an error signal fed to a servoamplifier to control a metering valve.
U.S. Pat. No. 3,372,590 to Sterling discloses a thermal flow meter employing a constant current source having a transistor 26 biased at its base by a Zener diode connected to its base. However, it should be noted that the flow meter of Sterling supplies an output voltage having a nonlinear relationship with respect to the rate of fluid flow in conduit 90, as is clearly shown in FIG. 7.
A mass flow controller is disclosed herein having a Zener diode biased constant current generator, which supplies a constant current to a bridge having an upstream resistor leg and a downstream resistor leg positioned in good heat conduction relation with a branch conduit to thermally detect the rate of flow of a gas traveling through the conduit. A variable gain bridge amplifier having good D.C. response and minimal high frequency gain is connected to the bridge to produce an amplified sensor voltage signal indicative of the relative difference in temperatures of the upstream and downstream arms of the bridge in response to the rate of flow of the gas through the branch conduit. The bridge amplifier includes a feedback loop having a user adjustable impedance to provide a user selectable gain for the bridge amplifier.
A plurality of operational amplifiers configured as precision limiters are connected in parallel with one another and receive an output signal from the bridge amplifier. Each of the precision limiters is adjusted during assembly to provide piecewise linearization of the bridge amplifier signal.
A comparator having limited high frequenty gain receives a set point voltage signal and the piecewise linearized signal and provides a comparator output signal having an amplitude related to the difference between the linearized signal and the set point signal.
The comparator output signal is supplied to a servoamplifier which drives a solenoidal control valve. A feedback loop having capacitive reactance is connected from a servoamplifier output to an input terminal of the comparator where a feedback signal is added to the linearized signal.
It is a principal object of the present invention to provide a mass flow controller having a plurality of operational amplifier precision limiters for conditioning a sensor signal to provide a linearized signal which is substantially linear with respect to the rate of flow of a gas.
Other objects of the invention will become obvious to one skilled in the art upon a perusal of the specification and claims in light of the accompanying drawings.